This invention relates to transmitters in general and more particularly to a VSWR protection circuit which exhibits high immunity to electromagnetic interference or EMI.
Essentially, the voltage standing wave ratio VSWR is an undesirable effect, and when it exists in transmitters, it may be harmful to the operation of the system. Standing waves caused by reflection from impedance discontinuities some where along a transmission line or in the coupling between an antenna and a transmitting amplifier is an example of an interference phenomenon between two coherent waves traveling in opposite directions. It is the periodic constructive and destructive interference between the incident and reflective waves which creates the standing wave pattern.
Minima occur every half wavelength where the incident and reflected waves are out of phase and subtract, while maxima occur where the two waves are in phase and add. The voltage standing wave ratio or VSWR is the ratio of these two quantities. Hence when a transmitting amplifier is perfectly matched and terminated, there is no reflective wave, and hence there is no standing wave, and the VSWR reduces to unity. For the case of total reflection, the VSWR would be infinite meaning zero voltage at the standing wave minima, while the maximum voltage would be twice the source voltage. This condition is approximated in a short circuited transmission line.
In any event, it is extremely desirable in a transmitter to attempt to match the transmitting amplifier to the antenna to thereby produce no reflected signals from the antenna. In all practicality this is not possible as there are discontinuities in both circuits. One way of attempting to solve this problem in the prior art is to monitor the amount of power reflected from the antenna back into the transmitting amplifier and detect the power and use the detected signal as a feedback control or an automatic gain control for the transmitting amplifier. This technique attempts to vary the gain so as to limit the amount of power reflect back into the transmitter.
Many transmitter configurations are designed to work with a 2:1 VSWR, annd such transmitters suffer approximately a 2.5 DB reduction in power if the standing wave ratio is increased to a 2.5:1 VSWR. Conventional prior art VSWR protection of solid state high power transmitters does not allow for the discrimination between reflected power due to the load VSWR which is the antenna VSWR and reflected power from adjacent transmitter units. As one will ascertain, an antenna is a bidirectional device and is capable of transmitting or receiving power. Hence when a transmitter is operating in the vicinity of another transmitter which is radiating at an output frequency close to that of the first, the antenna of the first transmitter will, in fact, receive the signal frequency from the other transmitter. The receipt of this signal, based on prior VSWR compensating techniques appears as a radiated signal and hence the circuitry employed to limit reflected power receives a larger signal than the transmitter is actually transmitting. Thus the gain of the transmitting amplifier is substantially reduced. The reduction in the gain of the transmitting amplifier causes the output power to decrease accordingly, and hence in the presence of a high power transmitter, an adjacent transmitter may actually shut down or produce output power which is totally acceptable.
Thus the conventional VSWR protection of high power transmitters does not allow for the discrimination between the reflected power due to the low VSWR and reflected power from an adjacent transmitter unit. These prior art techniques work well only in a situation where no reflected power from other transmitter units (EMI) is present. VSWR protection of solid state transmitters or other transmitters is a very desirable feature. In the typical tactical environment, many sources of electromagnetic interference (EMI) are present which can cause conventional VSWR protection loops to respond erroneously effectively reducing the transmitter output power to a very low and unacceptable value.
It is therefore an object of the present invention to provide an improved VSWR protection circuit.
It is a further object of the present invention to provide a VSWR protection circuit which will discriminate between reflected power at the transmitted frequency and EMI reflected power.